1. Field of the Invention
This invention relates to variable power finders, and more particularly to real image type variable power finders of simple structure suited to be used in photographic cameras or video cameras, while still preserving good optical performance before and after the magnification of the finder is varied.
2. Description of the Related Art
For the camera whose photographic and finder systems have nothing in common, when the photographic system is variable in power, it has been desirable from the standpoint of quick and easy shootings to make variable the angular field of the finder, or the display of the shooting area within the viewfield with variation of the power.
In the past, as the means for changing over the display of the shooting area there have been known:
(a) the method of varying the size of the viewfinder frame;
(b) the method of varying the magnification of the finder; and others.
An example of the method of varying the size of the viewfinder frame, as in Japanese Laid-Open patent application No. Sho 54-161931, is by moving a member for forming the image of a viewfinder frame to change the apparent size of the viewfinder frame. In this case, however, as the magnification of the finder increases, or the angular field of the finder narrows, the apparent angular field of the viewfinder frame becomes so small as to give a disadvantage that, for the observer, the finder becomes very uncomfortable to view through.
In the method of varying the finder magnification itself, as, for example, disclosed in Japanese Laid-Open Utility Model patent application No. Sho 59-168738 assigned to the assignee of the present invention, where the objective lens constituting part of an Albada finder is interchanged with another one having a different refracting power, on the other hand, the apparent angular field of the finder can be maintained constant. For the observer, therefore, it becomes possible to view through the finder with a closer sense to the actual shooting sense. Hence, it can be said that the latter method is superior to the former.
Besides this, there is another or real image type finder whose objective lens is positive in refracting power, or the so-called Kepler-type finder. Because of its ability to put a field stop mask in coincidence with the real image plane, this finder gives a clear contour to the viewfield frame. Another merit is that, despite the fact that position of the observer's eye changes, the field of view of the finder is not caused to change. For these reasons, the Kepler-type finder has round its use in many variable power finders. However, it has a disadvantage that the size of the finder system is liable to increase due to the formation of the real image of the viewfield. To preserve high grade of optical performance, the complexity of structure of the finder system also tends to increase. This tendency has become prominent when a variable power capability is introduced to the Kepler-type finder.
For example, the Japanese Utility Model Patent Publication No. Sho 29-16556 provides the technique of varying the power by interchanging the objective lens as a whole. But, the employment of this technique led to a large number of lens elements to be used.
Also, the introduction of the zoom function to the real image type finder as in the Japanese Laid-Open patent application No. Sho 61-156010 necessitates an operating mechanism of so high precision accuracy that the positions and speeds of a plurality of lenses can be sufficiently controlled well.